Thyroid peroxidase (TPO) is a thyroid-specific enzyme that displays complex antigenicity in autoimmune thyroid diseases. Previous studies from my laboratory have used recombinant forms of human TPO to identify autoantibody and T cell recognition sites in TPO. In one major region (amino acids 570-633), three autoantibody epitopes were identified that displayed mutually-exclusive binding by sera from patients with thyroiditis. One of these epitopes appears to be species-specific for human TPO, and does not share essential amino acid residues with potentially cross-reactive molecules such as lactoperoxidase. Autoantibodies to this area are preferentially associated with thyroiditis as compared to Graves' disease, and are specific to patients with autoimmune disease as they are not observed in patients with thyroid cancer or in normal individuals. Based on this information, it is hypothesized that there are unique immune responses to TPO that relate to the development or propagation of autoimmune thyroid disease. Understanding the immune response to TPO will help to clarify the reasons why an initial response to a self antigen develops and how this response might lead to an autoimmune disease. To characterize these responses, the exact epitopes recognized by antibodies and T cells involved in autoimmune recognition of TPO will be determined and the clinical significance of these responses will be analyzed. These epitopes will be examined to identify potential secondary and tertiary molecular structure and critical amino acid residues required for immune recognition. The sequences and theoretical structures of these epitopes will be compared with those of related peroxidase enzymes, especially those potentially involved in the induction of the autoimmune response (such as lactoperoxidase), to determine the specificity of the autoantibody recognition for TPO. In addition, the genetic basis for the autoantibody response to specific TPO epitopes will be characterized. The immunoglobulin VH clonality and usage in the antibody responses to different epitopes in each individual, and the responses to similar epitopes in different individuals will be compared to determine the restriction of immune responses to TPO. An examination of the nature of the T cell response in terms of TH1 vs. TH2 subsets and cytokine production will be performed. Finally, the clinical significance of autoimmune responses to each particular epitope in TPO will be examined in patients with the different disorders in which auto-reactivity to TPO has been demonstrated. An additional goal is to develop an understanding of the role of autoantibodies in the pathogenesis of autoimmune disease. The results of these studies should help to clarify the basis of TPO recognition by autoantibodies and begin to examine the unique facets of the T cell response against TPO that lead to autoimmune disease. Identification of specific TPO epitopes with clinical significance will provide better assays for the diagnosis and management of autoimmune thyroid disease. Information gained from these studies should also provide insights that can be generalized to other autoimmune diseases.